Alumina-zirconia ceramic

ABSTRACT

A ceramic consisting essentially of from 1 to 15 percent of glass and 99 to 85 percent of a mixture of particulate Al2O3 and particulate ZrO2 is disclosed. ZrO2 is present in a sufficient amount usually from 1/4 to 6 percent base on the of the ZrO2 and Al2O3, to strengthen the ceramic significantly, by comparison with an otherwise identical ceramic where the particulate ZrO2 is replaced either by the glass or by particulate Al2O3. The glass constitutes a vitreous phase bonding the particulates into a dense, gas impervious structure, and can be a calcium magnesium silicate glass containing from 45 to 80 percent of SiO2, from 8 to 55 percent of CaO and MgO, and not more than 15 percent of Al2O3.

REFERENCE TO RELATED APPLICATION

This is a continuation in part of application Ser. No. 893,609, filed Aug. 6, 1986, now U.S. Pat. No. 4751,207 granted June 14, 1988.

DEFINITIONS

As used herein, and in the appended claims, the terms "percent" and "parts" refer to percent and parts by weight, unless otherwise indicated; g means gram or grams; cm means centimeter or centimeters; and MPa means 10⁶ Pascals.

All temperatures herein are in degrees C., unless otherwise indicated.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an alumina ceramic containing a small, strengthening addition of zirconia; the ceramic is one which can be produced by a method that lends itself to mass production.

2. The Prior Art

The subject of alumina-zirconia ceramics produced both by hot pressing and by sintering techniques has received a great deal of attention during recent years. A journal article, Cer. Eng. and Sci. Proc., Vol 1,7-8(B) 1980, is considered to be typical of the prior art relating to such ceramics made by hot pressing. The article, D. Lewis III and P.F. Becher, "Thermal Shock Behavior in Al₂ O₃ -based composites", reports test data indicating that aluminazirconia composites which were studied are highly resistant to thermal shock. The data relate to alumina ceramics and to ceramics composed of alumina and up to 30 percent by volume of ZrO₂ produced by hot pressing at 1500-1600 and 35 MPa (about 5075 pounds per square inch). The data presented indicate the alumina-zirconia ceramics to have outstanding thermal shock properties. Another journal article, J.Am.Cer.Soc., 61, No. 12, pp. 85, 86, and U.S. Pat. No. 4,218,253, are illustrative of the prior art relating to the production of such ceramics by sintering. The patent discloses (Example 1) the production of an aluminazirconia ceramic from aluminum oxide powder (average particle size 5 microns) and monoclinic zirconium oxide powder (average particle size 1 micron). The process involves wet mixing the two powders, drying and granulating the mixture, isostatically pressing a shape from the granules, and sintering the shape at 1600 for one hour. The journal article discloses a similar process, including sintering at 1500 and 1600, but is silent as to particle size, disclosing only that "composites with a very fine and homogeneous dispersion" were achieved "by a wet-chemical method, starting from a zirconium sulfate-aluminum sulfate solution." It will be appreciated that hot pressing alumina-zirconia ceramics at 1500-1600 and 35 MPa is a costly procedure, that even sintering at 1600 is costly, and that alumina produced by a wet chemical method from a zirconium sulfate-aluminum sulfate solution is a costly starting material. Accordingly, as might be expected, the ceramics produced by the methods of the subject references, and all other alumina-zirconia ceramics that have been suggested by the known prior art, are costly and have found only limited commercial use, for example as tool bits.

Japanese patent application No. 53-126008, Nov. 2, 1978, discloses the production of what has been translated as an "aluminum ceramic" having improved thermal shock resistance by pressing discs from ceramic batches and firing to 1065°. Batches containing 94 to 96 parts by weight of Al₂ O₃, 1 to 2 parts by weight of CaO, 1 part by weight of MgO, 2 to 3 parts by weight of SiO₂, up to 2 parts by weight of Cr₂ O₃ and up to 10 parts by weight of ZrO₂ are disclosed.

The present inventor discovered a ceramic composed of a mixture of particulate Al₂ O₃, particulate ZrO₂ and glass bonding the Al₂ O₃ and the ZrO₂ into a dense, gas impervious structure, and also found that all or any part of the particulate ZrO₂ in such ceramics can be replaced by particulate HfO₂ or by a solid solution of HfO₂ and ZrO₂, in particulate form, as well as that Y₂ O₃ can advantageously be present to stabilize at least a part of the ZrO₂, the HfO₂ or the ZrO₂ -HfO₂ solid solution in a cubic crystalline structure. The glass constitutes from 1 to 15 percent of such ceramics, while particulate Al₂ O₃ constitutes from 75 to 85 percent, based upon the weight of the ZrO₂, HfO₂, Y₂ O₃ and Al₂ O₃. "Manning", U.S. Pat. No. 4,552,852, is directed to this discovery.

SUMMARY OF THE INVENTION

The instant invention is based upon the discovery of a ceramic which consists essentially of from 1 to 15 percent of glass and from 99 to 85 percent of a mixture of particulate Al₂ O₃ and particulate ZrO₂ in which particulate ZrO₂ is present in a sufficient amount, usually from 1/4 to 6 percent based on the weight of the ZrO₂ and Al₂ O₃, to strengthen the ceramic significantly, by comparison with an otherwise identical ceramic where the particulate ZrO₂ is replaced either by the glass or by particulate Al₂ O₃. The particulates should have an ultimate particle size finer than 15 microns. The glass constitutes a vitreous phase bonding the particulates into a dense, gas impervious structure and, preferably, is a calcium magnesium silicate glass containing from 45 to 80 percent of SiO₂, from 8 to 55 percent of CaO and MgO, and not more than 15 percent of Al₂ O₃. Preferably, the glass constitutes from 3 to 12 percent of the ceramic, most desirably from 5 to 12 percent. Available data indicate that a large increase in strength is achieved when larger addition of ZrO₂, e.g., 1/4 percent based upon the weight of Al₂ O₃ and ZrO₂ in the ceramic, is made, and that little if any additional increase in strength is achieved when a small additions, e.g. 1/2 percent on the indicated basis, are made. Since ZrO₂ is an expensive constituent of ceramics of the type in question, it is desirable to minimize the quantity used. Accordingly, ZrO₂ preferably constitutes from 1/4 to 4 percent, most desirably from 1/2 to 11/2 percent, based upon the weight of the Al₂ O₃ and ZrO₂ therein, in a ceramic according to the instant invention. In fact, the greatest benefit from the expensive batch ingredient is achieved when ZrO₂, on the indicated basis, ranges from 1/2 to less than 1 percent.

As is indicated above, Manning discloses that particulate HfO₂ and solid solutions of HfO₂ and ZrO₂ can be substituted for ZrO₂ in ceramics of the type in question, and that Y₂ O₃ can also be used, the purpose of the Y₂ O₃ being to stabilize the ZrO₂ or the like in a cubic crystalline structure. The same is probably true in ceramics according to the instant invention, but there is ordinarily no reason for such a ceramic to contain more than the amount of HfO₂ that is introduced thereinto by the 1 to 3 percent thereof that is present as an impurity in ZrO₂ as it occurs in nature. Accordingly, a ceramic according to the invention consists essentially of particulate Al₂ O₃, particulate ZrO₂ and glass, but, in accordance with the recognized meaning of the recitation, may contain HfO₂, Y₂ O₃ or other incidental ingredients so long as they do not interfere with the strengthening that is achieved because of the presence of the ZrO₂.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following examples are presented solely for the purpose of further illustrating and disclosing the invention. They are to be construed as illustrative, and not as limiting. Example 2 constitutes the best mode presently contemplated by the inventor.

EXAMPLE 1

A ceramic batch composed of 0.84 part of ZrO₂, 82.79 parts of Al₂ O₃, 16.07 parts of clays and other fluxes and 0.30 part sodium lignosulfonate was wet milled for 8 hours, 70 percent solids in water, in a 17-liter mill jar. The resulting batch was spray dried. The spray dried batch was then pressed isostatically at about 5500 psi. (about 38 MPa) in a cylindrical mold; the pressed shape was rotated and ground by contact with a rotating grinding wheel to a diameter of about 3.8 cm.; and the body was placed in a setter and fired in a gas fired tunnel kiln (slightly reducing atmosphere): about seventeen hours from ambient temperature of about 22 to 1545, and about 1 1/5 hours from 1545 to 1550, followed by cooling. The fired insulator which resulted had a diameter of about 2.5 cm. The mean Charpy Impact Strength of machined bars cut from insulators produced as described in this Example was 1.68× 10⁻³ foot pounds.

The ZrO₂ used as described above in Example 1 is commercially available from TAM Ceramics, Inc. under the trade designation "Zirox 360". It consists of ZrO₂, 1 to 3 percent of HfO₂, assay, ZrO₂ and HfO₂ at least 98.5 percent, and incidental impurities. A sample from the shipment from which ZrO₂ was used as described herein was found to have a median particle diameter of 9.71 microns and a specific surface area of 0.97 m² /cm³.

The Al₂ O₃ used as described above in Example 1 is commercially available from Aluminum Company of America under the designation A-10 alumina. Substantially all of the material is minus 10 microns in ultimate particle size, the median ultimate particle size being 8 microns; the material is agglomerated, however, so that it has the following size characteristics relative to screens of the U.S. Sieve Series:

100 mesh 4-15 percent retained

200 mesh 50-75 percent retained

325 mesh 88-98 percent retained and 2-12 percent through.

The material consists of Al₂ O₃, assay 99.5 percent, and incidental impurities.

The sodium lignosulfonate used as described above in Example 1 is a water soluble dispersing agent which is commercially available from American Can Company under the designation "MARASPERSE".

The clays and other fluxes used as described above in Example 1 contain SiO₂, MgO, CaO and Al₂ O₃ in such proportions that the fired insulator contained 89.44 percent of Al₂ O₃, 0.86 percent of ZrO₂, 7.02 percent of SiO₂, 1.56 percent of MgO, 0.71 percent of CaO and 0.41 percent of incidental impurities. A minor amount of the Al₂ O₃ was dissolved in a glass which also contained the SiO₂, the MgO and the CaO. The glass constituted about 10 percent of the fired insulator. The precise amount of Al₂ O₃ dissolved in the glass was not determined but, on the basis of available phase data, it was estimated that Al₂ O₃ constituted less than 5 percent of the glass.

EXAMPLES 2 and 3

The procedure of Example 1 was repeated to produce insulators from different ceramic batch compositions. The compositions of the ceramic batches in parts and the mean Charpy Impact Strengths are given in the following table:

    ______________________________________                                                   Control  Example 2 Example 3                                         ______________________________________                                         ZrO.sub.2   0.00       0.42      1.26                                          Al.sub.2 O.sub.3                                                                           83.63      83.21     82.37                                         Clays and   16.07      16.07     16.07                                         other fluxes                                                                   Sodium ligno-                                                                              0.30       0.30      0.30                                          sulfonate                                                                      Charpy Impact                                                                              1.07       1.82      1.82                                          Strength,                                                                      footpounds,                                                                    × 10.sup.3                                                               ______________________________________                                    

EXAMPLES 4-7

Insulators were also produced from other ceramic batch compositions using substantially the procedure of Example 1, the only differences being that the batches were milled in 7-liter mill jars, 70 percent solids in water, for 10 hours; and 0.39 part ammonium polyelectrolyte dispersant was used in place of the sodium lignosulfonate. The compositions of the ceramic batches in parts and the mean Charpy Impact Strengths are given in the following table:

    ______________________________________                                                      Example  Example  Example                                                                               Example                                  Control      4        5        6      7                                        ______________________________________                                         ZrO.sub.2                                                                              0.00     0.84     1.67   2.50   4.18                                   Al.sub.2 O.sub.3                                                                       83.55    82.71    81.88  81.05  79.37                                  Clays and                                                                              16.06    16.06    16.06  16.06  16.06                                  other fluxes                                                                   Ammonium                                                                               0.39     0.39     0.39   0.39   0.39                                   poly-                                                                          electrolyte                                                                    dispersant                                                                     Charpy  1.824    2.448    2.309  2.040  2.063                                  lmpact                                                                         Strength,                                                                      footpounds,                                                                    × 10.sup.3                                                               ______________________________________                                    

EXAMPLES 8-12

Insulators were also produced from other ceramic batch compositions using substantially the procedure of Example 1, the only differences being that the batches were milled in 7-liter mill jars, 72 percent solids in water, for 91/2 hours. Control insulators were produced from a batch composed of 83.24 parts of Al₂ O₃, 15.98 parts of clays and other fluxes, 0.25 part of sodium lignosulfonate and 0.53 part of ammonium polyelectrolyte dispersant, while test insulators were produced from batches composed of 82.75 parts of Al₂ O₃, 15.98 parts of clays and other fluxes, 0.25 part of sodium lignosulfonate, 0.53 part of ammonium polyelectrolyte dispersant and 0.49 part of various grades of zirconia. The grades of zirconia tested, the Charpy Impact Strength in footpounds ×10³ of machined bars cut from insulators made from each of the batches, and the fired bulk density in grams per cubic centimeter of the insulators, and the cantilever breaking load in pounds of the terminal ends of spark plug insulators made from each of the batches are set forth in the following table:

    __________________________________________________________________________                   Example                                                                             Example                                                                             Example                                                                             Example                                                                             Example                                               Control                                                                             8    9    10   11   12                                           __________________________________________________________________________     Zirconia --   "Zirox                                                                              "SC- "Zirox                                                                              "HSY-3"                                                                             "DK-1                                        Grade         360" 101" Tr"                                                    Charpy   1.140                                                                               1.940                                                                               1.750                                                                               2.010                                                                               1.760                                                                               1.690                                        Impact                                                                         Strength                                                                       Fired Bulk                                                                              3.524                                                                               3.569                                                                               3.571                                                                               3.571                                                                               3.575                                                                               3.571                                        Density                                                                        Cantilever                                                                              465  534  505  503  526  543                                          Breaking Load                                                                  __________________________________________________________________________

Grade "SC-101" of zirconia is commercially available from Magnesium Elektron. It was found to have a median particle diameter of 4.14 microns and a specific surface area of 1.47 m² /cm³. It consists of ZrO₂, 1-3 percent of HfO₂, assay, ZrO₂ and HfO₂ at least 98.5 percent, and incidental impurities.

Grade "Zirox Tr" of zirconia is commercially available from TAM Ceramics, Inc. It was found to have a median particle diameter of 3.43 microns and a specific surface area of 1.49 m² /cm³. It consists of Zro₂, 1-3 percent of HfO₂, assay, ZrO₂ and HfO₂ at least 98.5 percent, and incidental impurities.

Grade "HSY-3" of zirconia is commercially available from Daiichi Kigenso. It was found to have a median particle diameter of 3.37 microns and a specific surface area of 1.67 m² /cm³. It consists of ZrO₂, 1-3 percent of HfO₂, assay, ZrO₂ and HfO₂ at least 93.1 percent, 5.4 percent of Y₂ O₃, and incidental impurities.

Grade "DK-1" of zirconia is commercially available from Daiichi Kigenso. It was found to have a median particle diameter of 3.37 microns and a specific surface area of 1.76 m² /cm³. It consists of ZrO₂, 1-3 percent of HfO₂, assay, ZrO₂ and HfO₂ at least 98.5 percent, and incidental impurities.

It will be observed from a comparison of the foregoing data concerning Examples 1-3 and the associated control with the data concerning Examples 4-7 and the associated control and that for Examples 8-12 and the associated control that the different milling procedures used caused substantial changes in the magnitudes of the Charpy Impact Strengths. The percentage increases, however, relative to the relevant controls, follow the same patterns, indicating that the strengthening caused by the added ZrO₂ is independent of the milling procedures. The percentage increases are reported in the following table:

    ______________________________________                                                  Percent of Added ZrO.sub.2                                                     0.5 1.0      1.5   2.0    3.0 5.0                                     ______________________________________                                         Example 2  70                                                                  Example 8  70                                                                  Example 9  54                                                                  Example 10 76                                                                  Example 11 54                                                                  Example 12 48                                                                  Example 1        57                                                            Example 4        34                                                            Example 3                 70                                                   Example 5                     27                                               Example 6                            12                                        Example 7                                13                                    ______________________________________                                    

EXAMPLES 13-16

Insulators having a nominal 92 percent Al₂ O₃ content, and containing varying amounts of ZrO₂ were also produced using substantially the procedure of Example 1, the only differences being that a different grade of Al₂ O₃ was used and that the batch was milled in 7-liter mill jars, 72 percent solids in water, for 101/2 hours. Typical batch compositions, Charpy Impact Strengths and densities are given in the following table:

    ______________________________________                                                      Example  Example  Example                                                                               Example                                  Control      13       14       15     16                                       ______________________________________                                         ZrO.sub.2                                                                              0.00     0.25     0.49   0.73   0.97                                   Al.sub.2 O.sub.3                                                                       87.49    87.49    87.49  87.49  87.49                                  Clays and                                                                              11.73    11.48    11.24  11.00  10.76                                  other fluxes                                                                   Sodium  0.25     0.25     0.25   0.25   0.25                                   ligno-                                                                         sulfonate                                                                      Ammonium                                                                               0.53     0.53     0.53   0.53   0.53                                   polyelec-                                                                      trolyte                                                                        dispersant                                                                     Charpy  1.32     l.70     1.94   1.96   1.84                                   Impact                                                                         Strength,                                                                      footpounds                                                                     × 10.sup.3                                                               Fired Bulk                                                                             3.616    3.634    3.642  3.644  3.651                                  Density,                                                                       g/cm.sup.3                                                                     ______________________________________                                    

The Al₂ O₃ used in the batches of the previous table was obtained from Aluminum Company of America under the grade designation A-121. Substantially all of the material is minus 5 microns in ultimate particle size; the material is agglomerated, however, so that it has the following size characteristics relative to screens of the U.S. Sieve Series:

100 mesh 4-15 percent retained

200 mesh 50-75 percent retained

325 mesh 88-98 percent retained and 2-12 percent through

The material consists of Al₂ O₃, assay 99.5 percent, and incidental impurities.

It will be noted from the data in the foregoing table that both the fired bulk densities and the strengths of the specimens produced were increased by the additions of ZrO₂ that were made in the procedures of Examples 13-16. This is generally true of ceramics according to the instant invention. Although the invention is in no way to be limited by the following theory, it is believed that the ZrO₂ additions cause a decrease of the size of the pores of the ceramic, and that both the increased density and the strengthening are consequences of this decrease.

It will be apparent that various changes and modifications can be made from the specific details of the invention as described above without departing from the spirit and scope thereof as defined in the appended claims. 

What I claim is:
 1. A ceramic consisting essentially of from 1 to 15 percent of glass and 99 to 85 percent of a mixture of particulate Al₂ O₃ and particulate ZrO₂, the particulates all being finer than 15 microns in ultimate particle size, ZrO₂ constituting from 1/4 to 4 percent based upon the weight of ZrO₂ and Al₂ O₃ in the ceramic, and said glass constituting a vitreous phase bonding the particulates into a dense, gas impervious structure, and being a calcium magnesium silicate glass containing from 45 to 80 percent of SiO₂, from 8 to 55 percent of CaO and MgO, and not more than 15 percent of Al₂ O₃.
 2. A ceramic as claimed in claim 1 wherein the glass constitutes from 3 to 12 percent thereof.
 3. A ceramic as claimed in claim 1 wherein the glass constitutes from 5 to 12 percent thereof.
 4. A ceramic as claimed in claim 1 wherein particulate Al₂ O₃ constitutes more than 99 and not more than 991/2 percent, based upon the weight of the ZrO₂ and Al₂ O₃ in the ceramic.
 5. A ceramic as claimed in claim 2 wherein particulate Al₂ O₃ constitutes more than 99 and not more than 991/2 percent, based upon the weight of the ZrO₂ and Al₂ O₃ in the ceramic.
 6. A ceramic as claimed in claim 3 wherein particulate Al₂ O₃ constitutes more than 99 and not more than 991/2 percent, based upon the weight of the ZrO₂ and Al₂ O₃ in the ceramic. 